Aircraft: B757
Phase of flight: Initial Approach
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT

I was pilot flying into MTPP. Weather was day VMC with good visibility. Captain and I had briefed that we would fly the approach to [Runway] 10; then circle to land [Runway] 28. We discussed circling altitude and the fact that the glide path on the instrument approach to [Runway] 28 was steeper than average. Also [we] discussed that the PAPI's were hard to see above 1;000 FT; so the RNAV glidepath would be a back up once we were established on final. We were anticipating the DME arc to 10; and I believe it was over SAVAR that we were told to proceed direct IRDAS by ATC. The Captain did not hear ATC's garbled transmission clearly; and while ATC was phonetically spelling the name of the fix; I typed it in so as to turn in the correct direction. The Captain then programmed the remainder of the approach into the FMC. Between IRDAS and RIKOT I descended from 4;500 FT to 4;000 FT and we received a GPWS warning. Although I had visual contact with the terrain and saw no threat; I initiated a climb and the warning immediately ceased. Continued the approach and landed uneventfully.I believe that an earlier confirmation of the approach to be flown would have helped us to be better prepared. We did not expect to fly the full RNAV approach to [Runway] 28; and that put us behind in preparing to fly the approach. The difficulty of communications created a higher workload; between the Controller's heavily accented English and the poor quality of the radio transmissions overall. Being cleared direct IRDAS left a route discontinuity; since the transition was not selected and the intermediate step down altitudes between IRDAS and RIKOT were then not on the FMC. In the future; verify that all altitudes on the electronic chart are represented on the FMC. During a visual approach to MTPP with RNAV GPS 28 as a back-up; mis-programmed VNAV and began a descent from 4;500 FT over IRDAS Intersection to 3;000 FT early on terminal routing to the GPS final approach course. [We] got a momentary GPWS terrain warning while descending visually over terrain with terrain and airport in sight. Complied with the GPWS terrain warning and altered flight path immediately and warning ceased immediately. Continued descent to 3;000 FT to RIKOT Intersection and completed visual approach and landing Runway 28 MTPP. Pilot flying programmed direct to IRDAS Intersection per ATC and proceeded direct; after which myself; pilot not flying; cleared a discontinuity between IRDAS and RIKOT as pilot flying acquired the airport visually. As pilot not flying; I failed to look for and select a 'SAVAR' transition to the RNAV GPS 28 approach; which would have given the appropriate step-down points on the terminal routing using VNAV. Extenuating circumstances were garbled and weak radio transmissions from ATC and a late clearance directly to IRDAS intersection due to ATC mis-communication; subverting the 'SAVAR' transition. Also; crew was expecting and briefed a GANIV DME ARC arrival (which was on the flight plan) to a circling approach to Runway 28 using RNAV GPS 28 APP as a back-up. This was the procedure both pilots had experienced before and were expecting. Neither pilot had let-down to MTPP using direct routing to the RNAV GPS 28 approach previously.I believe that future recurrences of this event can be avoided by always looking for and selecting appropriate transitions; especially when being vectored or given direct to points inside the transition. Also; regardless of visual flight conditions; verify that the FMC glass routing AND ALTITUDES are the same as the iPad routing and altitudes before flying a procedure even as a visual back-up. B757 flight crew reports descending early on the RNAV Runway 28 to MTPP. The clearance direct IRDAS is received late and the transition from IRDAS is not selected in the FMC eliminating two step down fixes and results in a terrain warning.